Watts a horse power [RE-wrenches]

[Ray Walters]

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>
>Kurt, Todd,
>
>In a theoretical world of 100% efficiency, and by definition, 1 HP = 746 
>watts.
>
>Divide by the motor efficiency (expressed as a decimal) to get the actual 
>watts. For example:
>
>A high-efficiency motor (or motor/electronic system), like a solar pump 
>motor, may be in the range of 90% efficiency. So you divide 746 / .90 = 
>830 watts. (The difference goes off as heat.)
>
>An ordinary submersible pump motor is roughly 70% efficient. That's 
>because it's long and slender, which lengthens the windings beyond the 
>optimal configuration, and it's cheap, and there is no concern for the 
>heat since it's water-cooled. 746 / .70 = 1065 watts.
>
>So our beloved 1.5 HP pump would require about 1600 watts.
>
>Now figure about 20% more for the mod-square wave = 1920 watts.
>
>Now divide by the inverter efficiency, uh, maybe 85? so 1920 / .85 = 2260 
>watts.
>
>You measure over 2500 watts, but that's probably a little higher than 
>reality because the current draw of the inverter is pulsating and my guess 
>is that your meter probably reads it a bit high.
>
>So there you are. It works like a charm!   :-|
>
>
>Cheers (hopefully),
>Windy

We all almost forgot the ugly Power Factor in all of this, too. Inverters 
output is rated in VA, so a power factor of other than 1 means more 
inefficiency. I believe you would add another derating of around .6 or .7 
for a typical submersible (taken from my Franklin motor book)  Also, I 
don't think mod sine derating should be used in computing DC watts, only in 
total pumping efficiency. My understanding is that on mod sine, the pump 
turns slower and therefore pumps less water, not that it uses more 
electricity.

Ray
   

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